1. Field of the Invention
The present invention relates generally to electrical circuits and, more specifically, the present invention relates to electrical circuit clamping.
2. Background Information
Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency and good output regulation to power many of today's electronic devices. In a known switched mode power supply, a low frequency (e.g. 50 or 60 Hz mains frequency), high voltage alternating current (AC) is converted to high frequency (e.g. 30 to 300 kHz) AC, using a switched mode power supply control circuit. This high frequency, high voltage AC is applied to a transformer to transform the voltage, usually to a lower voltage, and to provide safety isolation. The output of the transformer is rectified to provide a regulated direct current (DC) output, which may be used to power an electronic device. The switched mode power supply control circuit usually provides output regulation by sensing the output and controlling it in a closed loop.
To illustrate, FIG. 1 is a schematic of a known forward power converter 101. A switch Q1 103 turns on and off in response to a control 105 to provide a regulated DC output voltage VOUT 129 from an unregulated DC input voltage VIN 127. In one embodiment, control 105 and switch Q1 103 are included in a switching regulator, which may be used to regulate the output voltage VOUT 129. This topology is well known and its operation is well documented.
Every forward converter must have a way to set the voltage on the primary winding 107 of the transformer 109 during the time when the switch Q1 103 is off. A popular way to set the voltage is with a clamp network 111 connected across the primary winding 107. The known clamp network 111 illustrated in FIG. 1 includes a resistor 113, a capacitor 115 and a diode 117 and absorbs and dissipates parasitic energy from the transformer 109 that is not delivered to the load 119 nor returned to the input 121. The balance of energy into the clamp network 111 through diode 117 and energy dissipated in 113 determines a clamp voltage VCLAMP 123 that is necessary prevent saturation of the transformer 109.
FIG. 2 shows with idealized waveforms how the voltage VSWITCH 125 on switch Q1 103 is related to the input voltage VIN 127 and the clamp voltage VCLAMP 123. The clamp voltage VCLAMP 123 must be high enough to prevent saturation of the transformer 109, but low enough to keep the voltage VSWITCH 125 below the breakdown voltage of switch Q1 103.
FIG. 3 shows the relationship between VCLAMP 123 and VIN 127 in a known power supply. As the input voltage VIN 127 changes, the clamp voltage VCLAMP 123 must be confined between the two boundaries shown in FIG. 3. The maximum voltage boundary is a straight line determined by the breakdown voltage of switch Q1 103. The minimum voltage boundary is a curved line determined by the voltage necessary to keep the transformer 109 from saturation.
FIG. 3 shows how the clamp voltage VCLAMP 123 behaves with an RCD network, such as that illustrated in clamp network 111 of FIG. 1. When the power converter 101 operates in continuous conduction mode, the clamp voltage VCLAMP 123 stays substantially constant in response to changes in VIN 127 at given load. The presence of leakage inductance in the transformer 109 causes the clamp voltage VCLAMP 123 to change with load 119. It is higher for greater current and lower for less current. The result is a restricted range of permissible input voltage VIN 127 that is shown in the shaded region of FIG. 3.